SQLite - lightweight RDBMS

SQLite is a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine and a Google-O'Reilly 2005 OpenSource Award winner!

Peter Bromberg's feedback - "It has an ADO.NET provider, as well as one for ASP.NET 2.0, and speed tests show it beats the pants off Firebird. In addition, its a single DLL that's only about 250K, and requires no installation."

Update:
SQLITE PRO Enterprise Manager is the equivalent of SQL Server Enterprise Manager for SQLite 3 databases.